The present invention relates to distance measuring devices for cameras, and more particularly, to a passive distance measuring device in which optical axes of a finder and those of optical elements of the distance measuring device are not aligned with each other.
Conventional distance measuring devices provided with automatic focusing systems (AF systems), often use a passive distance measuring device which utilizes external light. The passive distance measuring device of an automatic focusing system is mainly employed in a lens-shutter type camera in which a photographic optical system, a finder and a distance measuring optical system of the AF system are separately arranged. A brief description will subsequently be given of a lens-shutter type camera provided with a conventional distance measuring device with reference to FIGS. 1 through 3 inclusive. A photographic lens 12, a finder (objective window) 14, and a light emitting window 16 for a built-in strobe are provided on the front panel of a camera body 10 with a shutter release button 18 on the top surface thereof. Further, a pair of AF lenses 22, 23 of a distance measuring device 20 are disposed above the photographic lens 12 on the front panel of the camera body 10.
FIGS. 2 and 3 are a bottom and an elevational view of the distance measuring device 20, respectively. Before being projected on a distance measuring sensor 30, rays of light from the subject introduced from the pair of AF lenses 22, 23 are substantially inwardly reflected by respective mirrors 24, 25 at right angles and passed through respective condenser lenses 26, 27 to become incident upon a mirror prism 28 where they are rearwardly reflected at right angles.
The distance measuring sensor 30 is, as shown in an elevational view of FIG. 4, provided with a light receiving unit equipped with a CCD (Charge Coupled Device) line sensor 32 having a number of light receiving elements arranged in a row. The line sensor 32 consists of two sections 32A, 32B disposed in the row and the luminous flux of an object introduced from the pair of AF lenses 22, 23 is projected on the respective line sensors 32A, 32B. Each light receiving element of the line sensor 32 submits the projected image of the subject to photoelectric conversion and stores the image in the form of a signal charge. Numeral 34 denotes a monitor sensor for use in finding the optimum signal charge accumulation time for the line sensor 32.
A control system in the camera body is used for reading the signal charge stored in the light receiving elements of the line sensor 32, computing an object distance through operations, and driving a focusing lens up to a focusing position according to the measured value of distance.
A description will be given of the relation of a distance measuring zone to a finder field in the aforementioned camera with reference to FIG. 5. In this camera, the finder 14 is interlocked with the zooming of the photographic lens 12 to change its field magnification. If the portion of the object projected to the line sensor 32 on a finder field 36 is to be a distance measuring zone, the distance measuring zone at telephoto position becomes the zone as is shown in FIG. 5 as 37T. The finder field 36 is provided with a distance measuring frame 38 for visualizing the distance measuring zone. When the photographic lens 12 in this state is caused to zoom toward a wide angle, the field magnification of the finder 14 lowers but the size of the field frame 38 remains unchanged.
Despite the zooming, on the other hand, the magnification of the distance measuring device 20 also remains unchanged. As a result, a distance measuring zone 37W on the finder field 36 becomes small at a wide angle as shown in FIG. 5.
With reference to FIGS. 6A and 6B, a description will further be given of the relation between the finder field 36 and the distance measuring zone in cases of a telephoto and a wide angle when the same object is photographed from the same position.
At the telephoto position, it is assumed that an object image 39 on the finder field 36 and the distance measuring zone 37T are those illustrated in FIG. 6A. In this case, when the photographic lens 12 is zoomed toward the wide angle, the field magnification of the finder 12 lowers. Consequently, the object image 39 on the finder field 36 becomes smaller up to the size shown in FIG. 6B at the end of the wide angle.
On the other hand, the size of the distance measuring zone relative to the object does not vary since the magnification of the distance measuring device 20 remains unchanged as stated above. In other words, the size of the distance measuring zone relative to the object image 39 is constant. As shown in FIG. 6B, the distance measuring zone 37W on the finder field 36 has also becomes as small as the object image 39.
In the conventional distance measuring device 20, the size of the distance measuring zone on the finder field 36 would change as stated above as the field magnification of the finder 14 changes. In other words, because the size of the distance measuring zone occupying the finder field 36 varies with the focal length of the photographic lens 12, the problem is that an error in distance measuring may be made by measuring distance to an object not intended to be photographed by a photographer.
Moreover, the optical axis of the finder is separated from that of the distance measuring device in the conventional camera. As shown in FIG. 7, for instance, a pair of AF lenses 41, 42 of a distance measuring system and a finder 40 are provided substantially in a horizontal row as viewed from the front of the camera. A distance measuring frame 48 for visualizing a distance measuring zone 47 is provided in a finder field 46 of the finder 40 as shown in FIGS. 8A and 8B.
As stated above, however, the optical axis of the finder 40 is separated from those of the AF lenses 41 and 42. For this reason, the distance measuring frame 48 tends to shift from the actual distance measuring zone 47, depending on the subject distance. If the distance measuring zone 47 is arranged as to coincide with the distance measuring frame 48 at a standard range, for instance, the distance measuring zone 47 tends to shift to the right of the distance measuring frame 48 in the case of the subject located at a short distance (see FIG. 8A), whereas the distance measuring zone 47 tends to shift to the left of the distance measuring frame 48 in the case of the subject at a long distance.
When the AF lenses 41, 42 are disposed under the finder 40 as shown in FIG. 9, moreover, the distance measuring zone 47 tends to shift upward with respect to the distance measuring frame 48 in the case of the subject located at a short distance (FIG. 10A), whereas it tends to shift downward in the case of the subject at a long distance (FIG. 10B).
More specifically, since the optical axis of the finder is in coincident with that of the AF optical system, the distance of the subject shifted from the distance measuring frame 48 on the finder field 46 is being measured. The problem is therefore that the object which ought to have been photographed with the distance measuring frame 48 exactly focused thereon is found out of focus on a developed print.
A shift similar to what has been discussed above is amplified in a camera in which the optical axis of the finder deflects in the direction of the optical axis of the photographic lens because of the shifting of the finder field from the photographic image plane is corrected in macrophotography. As shown in FIG. 11, for instance, the optical axis of the finder 40 is caused to swing toward the photographic lens 49 in such a camera so that the finder 40 is transversely shifted towards the photographic lens 49. When the distance measuring frame 48 is coincident with the distance measuring zone 47 for standard photographs on the finder field 46 (FIG. 12A), the distance measuring zone 47 tends to shift to the left in macrophotography (FIG. 12B).
As shown in FIG. 13, moreover, the optical axis of the finder 40 is caused to swing down toward the optical axis of the photographic lens 49 in macrophotography when the finder 40 is provided above the photographic lens 49. As a result, even if the distance measuring zone 47 is coincident with the distance measuring frame 48 for standard photography time, it still poses a problem in that the former shifts from the latter (FIGS. 14A, for macrophotography).
In a camera equipped with a conventional distance measuring device, the photographic image plane or distance measuring zone on the finder field is caused to shift if the subject distance varies, since the optical axis of the photographic optical system has shifted from that of the AF optical system.
In addition to the problem stated above, another disadvantage is that the exclusive area of the photographic image plane or distance measuring zone on the finder field changes if the focal length changes when the photographic lens is a variable focal length lens such as a bifocal or zoom lens.
To solve the aforementioned first problem, there has been developed a means for forming the distance measuring sensor (line sensor) transversely longer (wider).
If however the line sensor is relied upon, the distance measuring zone becomes wider in the transverse direction. Then, as shown in FIG. 15, for instance, images of a plurality of portions of an object longitudinally spaced apart (i.e., a three-dimensional object) may be projected on the line sensor 32. In this case, the focus calculating operational means is unable to decide which one of the object images should be selected as a basis for performing a focusing operation, which also results in a problem that no correct object distance is available for use by the focusing mechanism.
When a camera equipped with the distance measuring device and a strobe is used to photograph a plurality of objects located different distances from the camera by means of the strobe, the quantity of light emitted by the strobe becomes inappropriate to the object if the strobe is focused on an object that is located outside the range of its use and this results in improper exposure. On the other hand, the properly exposed object will be out of focus. Thus, no objects are photographed properly.
In a camera equipped with an auxiliary projector for projecting a stripe pattern image onto a dark object or one that offers a low contrast to be photographed, the spot diameter (irradiation angle), if it has been adjusted to the distance measuring zone at a wide angle position, for instance, will become wider than the distance measuring zone at the telephoto position to increase wasteful irradiation. Thus long distance cannot be covered by the radiation of the auxiliary projector.